Feeding power to or from remote or awkwardly situated distributed installations from or to an existing power network creates slightly different challenges compared to normal installations. This is especially true for installations with several high power sources or loads. Examples of such remote or awkwardly situated distributed installations are offshore installations for oil and/or gas production especially installations sub sea, offshore wind mill installations, and mining districts or process plants at remote locations.
As is disclosed in EP 1385259, WO 0152379 and WO 0148892 it is favorable to use HVDC transmission to feed power to and from such installations.
The advantages of DC transmission have been commercially exploited since 1954 when the first HVDC transmission was commissioned. Mercury-arc valves were eventually replaced with high power thyristors and dc transmissions have reached several GW, over +/−600 kV, and distances around 1000 kilometres. In 1997, a new breed of HVDC converter stations and HVDC transmissions were introduced. ABB has named its product family HVDC Light®.
The use of forced-commutated power semiconductors in a voltage-source converter allows much increased control of the active power flow, reactive power flow and harmonics when connected to even weak ac grids. This is well-known from low-voltage applications. The key factor allowing voltage-source converters to be connected to networks at voltage levels hitherto unreachable, is the series-connection of power transistors—in ABB's case IGBTs.
HVDC Light® can be connected to underground/submarine cables or overhead lines, on the dc as well as the ac side. A significant difference between classic HVDC and HVDC Light® is that in the latter, the dc-link voltage polarity is constant, irrespective of the direction of power flow.
The present Light® technique, e.g. represented by EP 1385 259 A, is a good solution for remote or awkwardly situated distributed installations regarding controllability, low operating cost but also size and investment cost is within reasonable levels. In this solution, the power to the installation is fed through HVDC cables and a HVDC Light® converter operates as AC power supply for a local AC network used to distribute the power at the installation, e.g to electric motors etc. Back up power is often provided via a local power supply using gas turbines or the like. However, such local power supplies require considerably space and offshore manning.